Amplifier circuits are usually used in audio technology, for example in Class AB feedback amplifiers. In this case, the quality of the amplifier decisively depends on the gain which is supplied to the signal and in turn depends on the load that is connected to the output.
FIG. 1 shows the schematic structure of a conventional 3-stage Class AB feedback amplifier which essentially comprises a first amplifier stage 1 which is supplied with the input signal and forwards the latter to a second amplifier stage 2 in amplified form. From the output of the second amplifier stage, the signal which has been amplified once again passes through a Class AB regulating system 3 which is required for the output stage amplifier 4 and is connected to the output stage amplifier 4 that forwards the signal to the output.
In the field of audio technology, the output is usually connected to a loudspeaker which represents a complex load and whose drive frequency range is represented by an inductive component in the lower frequency range and by a capacitive component in the upper frequency range. The components of a loudspeaker 5, formed by a series circuit comprising a resistor R and an inductance L with a capacitance C connected in parallel with the series circuit, are illustrated in FIG. 2. An audio amplifier having such a complex load constitutes a particular challenge since the load behaves differently in different frequency ranges and a particularly high gain is required.
Since the load behaves like a capacitance at high frequencies, the non-dominant pole which decisively contributes to the stability is decisively determined by the gain factor gm of the output stage. In order to ensure the stability of the AC component of the amplifier circuit in all frequency ranges, the gain factor must generally satisfy the condition gm>>1/R, where gm is the gain factor, also called the transconductance, and R is the resistance of the loudspeaker. In typical Class AB amplifier output stages, the gain factor can be specified by gm=2*Iq/(Vgs,eff), where Iq represents the quiescent current and Vgs,eff represents the effective gate-source voltage. The AC stability of the amplifier is thus decisively achieved by a large quiescent current Iq caused by the DC component.
FIG. 3 shows the equivalent circuit diagram of a conventional output stage amplifier, the quiescent current Iq being passed from the supply voltage Vdd to the output via the PMOS part which comprises a p-channel transistor P. In a complementary manner to the PMOS part, the output is connected to a ground potential via the one NMOS part which comprises an n-channel transistor N.
In these output stage amplifier circuits, the stability of signal amplification is ensured at the expense of an increased total current draw which mostly comprises the quiescent current.